5 ms·

> With most classes, unless you design them really carefully, your methods are often at risk from being overridden by others in ways that don’t meet expectation

> With most classes, unless you design them really carefully, your methods are often at risk from being overridden by others in ways that don’t meet expectations — especially if it mutates the object and does so in a different way. WAT. Because Ruby and Python programmers are just bleeding to death from the thousand cuts of overriding methods during subclassing. Or... not? More seriously, is there something to Swift here that I'm missing? Because in just about any language I've run across with inheritance, either the superclass(es) are part of a framework that defines a protocol for the inheriting code (e.g. you may/must provide methods X, Y, and Z. These methods must behave according to the following rules {...}), or else the programmer writing the inheriting code takes on the full responsibility of integration.

overgard 11y ago
I can't speak for the author, but you could take it as an argument against inheritance in general. There are a lot of smart people in that boat -- I think at one point even James Gosling said he might have left inheritance out of java if he had it to do over again. I probably have no shot of convincing you that inheritance kind of sucks in a short post on a friday night, but I'd just point out that it's not an obscure opinion, there are a sizable number of people that really have no love for the "classic" inheritance model of OOP.
- cpeterso 11y ago
  Consider that neither Rust nor Go have traditional class inheritance. It's definitely out of favor these days. :)
  - saidajigumi 11y ago
    No doubt, and I'm not really debating the language design tradeoffs involved in various approaches to OOP (or heck, typing in general). It's just this kind of talk comes up from time to time, seems to be fear-driven, and ignores the pragmatic practices that have been developed to work with inheritance (or any number of other language features that trigger a similar reaction, c.f. Pythonistas vs Rubyists on the topic of monkey-patching). It was surprising enough in context of an otherwise incisive article that I was honestly wondering if there was something specific to Swift and/or the ObjC framework world that posed a unique hazard.
    - josephlord 11y ago
      Speaker here. Nothing particular although ObjC lacks any language based features to control inheritance. The concern is that if a method has a particular effect on state (possibly even private internal state) that you expect to take place that subclasses may override the method in such a way that it changes all the expectations and may leave you in an inconsistent state. This means that it is necessary to document (or have conventions for) what should and should not be overridden and whether overriding methods should call the superclass implementation too. In unsafe languages with raw pointers the issues may be more severe than languages like Python and Ruby but they are generally similar. I may have overstated the point too, I had notes but not a script. The point is that you need to be careful designing and documenting for subclassing.
- seanmcdirmid 11y ago
  It is not an obscure opinion at all. If anything, us inheritance lovers are on the defensive (the entire OOP community is, but that is another story).
  - Tloewald 11y ago
    There's an in-between option seen in some languages where you don't inherit and override but instead implement a superclass's stub (or event if you want to think of it another way), which I think gives almost all the benefits of inheritance without most of the downsides. Sadly it hasn't penetrated the "major" languages.
    - seanmcdirmid 11y ago
      That would negatively effect the encapsulation benefits of inheritance: we could never safely access private fields on any value other than "this." Try implementing binary methods without that! Inheritance should evolve...there is plenty that can be done with it (e.g. mixin-style inheritance, dynamic inheritance, family inheritance). At the very least, we should have that conversation on how to address inheritance criticisms.
platz 11y ago
he appears to be referring to the http://en.wikipedia.org/wiki/Liskov_substitution_principle http://en.wikipedia.org/wiki/Liskov_substitution_principle . Generally type systems don't enforce the invariants that subclasses have the power to break. It is the usually the duty of the programmer to ensure that subclasses don't break these invariants. Invariants aren't things like whether methods are overidden or not, it's things like whether area for a shape can be computed by just knowing a side, exemplified with the familiar case of whether a rectangle should be a subclass of a square, or the other way around
micv 11y ago
Anyone who inherits from a class and accidentally overrides a method is a hopeless case or a newbie needing basic review from a senior dev. The bugs are on them. They can either test that their inheritence works, learn from the mistake, or they can piss off and find a new career emptying bins, flipping burgers, or whatever they fancy that doesn't involve making sure that what they just did will work beyond the immediate function.
- overgard 11y ago
  > Anyone who inherits from a class and accidentally overrides a method is a hopeless case or a newbie needing basic review from a senior dev That's a pretty harsh stance. I don't mean to be a jerk, but how much professional experience do you have? Even in good code bases there are frequently complicated hierarchies with non-obvious dependencies that are very easy to screw up even by people who know what they're doing. Good teams find ways of automating the checking on these things so that they can spend their time thinking about other problems, or avoiding the complication altogether, rather than blaming the victim.
  - micv 11y ago
    I'm a decade in. I know what you mean about complex hierachies, but inheritence is a very, very dangerous tool; you either take the time to review the results, or you suffer the consequences. That's where the review by senior developers comes in. You learn or you don't. Who would want to work with devs who don't?
    - Jweb_Guru 11y ago
      I do, because I don't consider people's ability to deal with the vagaries of inheritance a valuable skill when you can just avoid it with effectively no negative consequences. Of course, to clarify, it's not really possible to avoid in languages like Java, where it is more or less mandatory due to the design of its standard library (unless you avoid that too, which is a considerably worse idea), but you can still keep your own CODE clean with liberal use of `final` and interfaces. Personally, I'm consistently baffled when programmers like yourself, who have come to appreciate that "inheritence [sic] is a very, very dangerous tool," continue to defend it so vigorously. If it offered really powerful or unique advantages, sure--but if you take a hard at what it actually provides, it's just syntactic sugar and a performance "optimization" (thin pointers) that's usually pessimal compared to other approaches. The downsides are significant and well-known (as mentioned above, Liskov substitution being undecidable is one consequence, but there are plenty of others). There's no point in making life harder for ourselves.

overgard 11y ago

I can't speak for the author, but you could take it as an argument against inheritance in general. There are a lot of smart people in that boat -- I think at one point even James Gosling said he might have left inheritance out of java if he had it to do over again. I probably have no shot of convincing you that inheritance kind of sucks in a short post on a friday night, but I'd just point out that it's not an obscure opinion, there are a sizable number of people that really have no love for the "classic" inheritance model of OOP.

cpeterso 11y ago
Consider that neither Rust nor Go have traditional class inheritance. It's definitely out of favor these days. :)
- saidajigumi 11y ago
  No doubt, and I'm not really debating the language design tradeoffs involved in various approaches to OOP (or heck, typing in general). It's just this kind of talk comes up from time to time, seems to be fear-driven, and ignores the pragmatic practices that have been developed to work with inheritance (or any number of other language features that trigger a similar reaction, c.f. Pythonistas vs Rubyists on the topic of monkey-patching). It was surprising enough in context of an otherwise incisive article that I was honestly wondering if there was something specific to Swift and/or the ObjC framework world that posed a unique hazard.
  - josephlord 11y ago
    Speaker here. Nothing particular although ObjC lacks any language based features to control inheritance. The concern is that if a method has a particular effect on state (possibly even private internal state) that you expect to take place that subclasses may override the method in such a way that it changes all the expectations and may leave you in an inconsistent state. This means that it is necessary to document (or have conventions for) what should and should not be overridden and whether overriding methods should call the superclass implementation too. In unsafe languages with raw pointers the issues may be more severe than languages like Python and Ruby but they are generally similar. I may have overstated the point too, I had notes but not a script. The point is that you need to be careful designing and documenting for subclassing.
seanmcdirmid 11y ago
It is not an obscure opinion at all. If anything, us inheritance lovers are on the defensive (the entire OOP community is, but that is another story).
- Tloewald 11y ago
  There's an in-between option seen in some languages where you don't inherit and override but instead implement a superclass's stub (or event if you want to think of it another way), which I think gives almost all the benefits of inheritance without most of the downsides. Sadly it hasn't penetrated the "major" languages.
  - seanmcdirmid 11y ago
    That would negatively effect the encapsulation benefits of inheritance: we could never safely access private fields on any value other than "this." Try implementing binary methods without that! Inheritance should evolve...there is plenty that can be done with it (e.g. mixin-style inheritance, dynamic inheritance, family inheritance). At the very least, we should have that conversation on how to address inheritance criticisms.

platz 11y ago

he appears to be referring to the http://en.wikipedia.org/wiki/Liskov_substitution_principle http://en.wikipedia.org/wiki/Liskov_substitution_principle . Generally type systems don't enforce the invariants that subclasses have the power to break. It is the usually the duty of the programmer to ensure that subclasses don't break these invariants. Invariants aren't things like whether methods are overidden or not, it's things like whether area for a shape can be computed by just knowing a side, exemplified with the familiar case of whether a rectangle should be a subclass of a square, or the other way around

micv 11y ago

Anyone who inherits from a class and accidentally overrides a method is a hopeless case or a newbie needing basic review from a senior dev. The bugs are on them. They can either test that their inheritence works, learn from the mistake, or they can piss off and find a new career emptying bins, flipping burgers, or whatever they fancy that doesn't involve making sure that what they just did will work beyond the immediate function.

overgard 11y ago
> Anyone who inherits from a class and accidentally overrides a method is a hopeless case or a newbie needing basic review from a senior dev That's a pretty harsh stance. I don't mean to be a jerk, but how much professional experience do you have? Even in good code bases there are frequently complicated hierarchies with non-obvious dependencies that are very easy to screw up even by people who know what they're doing. Good teams find ways of automating the checking on these things so that they can spend their time thinking about other problems, or avoiding the complication altogether, rather than blaming the victim.
- micv 11y ago
  I'm a decade in. I know what you mean about complex hierachies, but inheritence is a very, very dangerous tool; you either take the time to review the results, or you suffer the consequences. That's where the review by senior developers comes in. You learn or you don't. Who would want to work with devs who don't?
  - Jweb_Guru 11y ago
    I do, because I don't consider people's ability to deal with the vagaries of inheritance a valuable skill when you can just avoid it with effectively no negative consequences. Of course, to clarify, it's not really possible to avoid in languages like Java, where it is more or less mandatory due to the design of its standard library (unless you avoid that too, which is a considerably worse idea), but you can still keep your own CODE clean with liberal use of `final` and interfaces. Personally, I'm consistently baffled when programmers like yourself, who have come to appreciate that "inheritence [sic] is a very, very dangerous tool," continue to defend it so vigorously. If it offered really powerful or unique advantages, sure--but if you take a hard at what it actually provides, it's just syntactic sugar and a performance "optimization" (thin pointers) that's usually pessimal compared to other approaches. The downsides are significant and well-known (as mentioned above, Liskov substitution being undecidable is one consequence, but there are plenty of others). There's no point in making life harder for ourselves.